Bacterial release from pipe biofilm in a full-scale drinking water distribution system.
Chan S, Pullerits K, Keucken A, Persson KM, Paul CJ, Rådström P
NPJ Biofilms Microbiomes. 2019. doi: 10.1038/s41522-019-0082-9
COMMENT: Drinking water is delivered through pipes of drinking water distribution systems (DWDSs), these pipes are lined with biofilm that could affect water quality by releasing bacteria into the drinking water. This study describes the drinking water microbiome of a treatment plant in Varberg, Sweden. The installation of ultrafiltration (UF) facility reduced the total cell count in water leaving the treatment plant. With fewer bacterial cells in the distributed water, those originating from the pipe biofilm and released into the water could now be observed.
This study aims to quantify and describe the community of bacteria released from the pipe biofilm in a full-scale DWDS. The removal of the high background cell count by the installation of UF, removed the limitations in resolution for flow cytometry and 16S rRNA amplicon sequencing studies.
The short time frame in this study allowed the contribution of cells from the biofilm to be estimated as 0.5% of the total cells present in the water before the change. Applying this estimate for cells released from the biofilm to other systems where the bacterial concentration in the distributed water is high can explain why the contribution from the pipe biofilm to the water microbiome has been difficult to observe
The percentage of intact cells also increased in the water as it travelled through the DWDS, and may be a signature for bacterial release from pipe biofilm.
In the current study, lower diversity (due to both decreased richness and lower evenness) was observed for the community in distributed water after UF installation, compared to those in finished water and before UF installation.
Specific OTUs at class level accounted for much of the observed changes in the water microbiome, including Alphaproteobacteria and Nitrospira, which showed a higher relative abundance in the distributed water after the installation of UF.
Bacteria released from the biofilm were described by 29 OTUs where the absolute read abundance increased in the distributed water compared to the finished water.(…) .Six of the 29 OTUs released from the biofilm were classified as genus Nitrospira, a group of bacteria that has been found in bacterial communities in drinking water..
While numerous studies have associated Alphaproteobacteria, Sphingomonas, Nitrospira and Mycobacterium spp. with drinking water and its biofilms, this study showed that members of these classes and genera also move from the pipe biofilm into the drinking water.
Although the UF installation modified the type of organic matter and greatly reduced the number of bacterial cells in the distributed water, destabilization of the biofilm, observed as detachment, sloughing or a sudden increases in the number of total cells in distributed water, was not observed during the 114 days of the study.
Regions in the DWDS with longer retention times may gradually show increasing cell counts in distributed water from prolonged contact with the biofilm or the dynamics of bacterial release may change.
Since this study was conducted during winter, it is also not known to what extent the release of bacterial cells could change with increases in temperature or seasonal changes in water use.